In blasting operations, various arrangements of signal transmission lines, detonator units, delay elements and explosives are used to transmit a blast signal from a remote initiation location to explosives in boreholes. The explosive industry has moved away from detonating cord to low brisance transmission lines. Typically, there are two types of detonator units employed in blasting operations: the downhole unit and the trunkline and delay (T and D) unit. By using these two types of units, different blasting sequences and timing patterns can be arranged.
The downhole unit can be comprised of a length of signal transmission line with one end inserted into a detonator and the other end sealed. The detonator end of this unit is placed down a borehole to initiate a primer charge which initiates an explosive column in the borehole.
The T and D unit can also be comprised of a length of signal transmission line with one end inserted into a detonator and the other end sealed. The T and D unit is employed on the surface of the blast to initiate the signal transmission lines of one or more downhole units and/or other T and D units.
The detonator in the T and D unit can be placed inside a connection block. The detonator together with the connection block is a detonator assembly. The signal transmission lines of downhole units or other T and D units can be placed inside the connection block and compressed against the detonator. When the detonator discharges, the force of the discharge initiates these adjacent signal transmission lines.
In order to reduce noise in blasting operations, the industry has adopted the use of transmission lines, such as those illustrated in U.S. Pat. No. 4,290,366 to Janowski, that typically comprise a hollow tube containing a reactive element which transmits a detonation signal through the tube via a plasma wave. These transmission lines are virtually noiseless and produce no side blasts in contrast to previously employed detonating cord. Although initiation of an open end of the tubes is easily accomplished, initiating the tubes through their sides allows greater variety and simplicity in arrangement of the T and D and downhole units. Means to reliably initiate several signal transmission lines through their sides with one detonator is desirable.
The connection of two lines by connecting the detonator of one signal transmission line to the midpoint of another signal transmission line has been accomplished.
U.S. Pat. No. 3,987,733 to Spraggs et al. discloses a delay surface connector with a length of signal transmission line with a delay cap on each end and a protective block containing each delay cap. One protective block as disclosed has three longitudinal channels for holding one detonator cap and one signal transmission line folded double.
U.S. Pat. No. 3,987,732 to Spraggs et al. discloses a borehole downline unit that is essentially the same concept as Spraggs '733 except that one end of the signal transmission line has a high strength detonator for initiating borehole explosives.
U.S. Pat. No. 3,878,785 to Lundborg discloses an explosive assemblage similar to Spraggs '732 and '733. The connecting blocks as disclosed teach doubling the fuse through channels that run in close proximity to a cap.
Another approach to permit connection of a number of transmission lines involves the stacking of two or more connecting blocks each containing a detonator. U.S. Pat. No. 4,821,645 to Reiss discloses, among other things, a connector that has a well for receiving a blasting cap, one or more ports or channels for receiving a transmission line, and means for joining one connector with another connector. Because of the structure involved, this approach is useful for high strength caps.
Thus far, there has been a need in the industry to minimize shrapnel and noise by reducing the detonator size used in a T and D unit. Reduction in the size of the detonator has been hampered because a suitable connector design was not available which permitted connection of a differing number of transmission lines without loss of reliability.
Reliable initiation of signal transmission lines is a function of three factors: the strength of the detonator, the design of the detonator and the degree of confinement between the signal transmission line(s) and the detonator. If one or two factors are lacking, the other factor(s) must compensate. When the degree of confinement is increased and detonator output optimized by the detonator design, a higher percentage of the detonation energy is absorbed by the signal transmission lines. If a higher percentage of energy is being transmitted and then absorbed when there is increased confinement and optimized design of the detonator, then the strength of the detonator can be reduced and the actual amount of energy absorbed remains the same. It is desirable to reduce the strength of the detonator to decrease noise and shrapnel. The present invention involves an assembly that increases confinement and transfer of explosive energy such that a low strength detonator can be used to reliably initiate not only one signal transmission line but also a plurality of lines.
The detonator used in T and D units has been typically a No. 8 strength cap with 600 to 800 milligrams of secondary explosives as a base charge and 60 to 125 milligrams of primary explosives as a primer charge. Initiation of the No. 8 strength cap not only initiates the signal transmission lines to which it is operatively adjacent, but also completely destroys the connection block along with the entire detonator shell. This highly energetic and destructive detonation throws shrapnel from both the connection block and the detonator shell at very high velocities. This flying shrapnel has the potential of contacting the signal transmission lines of other units and either prematurely initiating them or cutting them such that they cannot transmit a signal. This is typically known as shrapnel "cut off."
Along with shrapnel cut offs, the No. 8 strength caps produce excessively noisy air blasts in excess of 140 db at three meters. In order to reduce the amount of air blast noise and shrapnel thrown by the T and D unit's detonator, the detonator assembly is often buried at the rim of the borehole. This activity requires extra time for completing the already arduous task of loading a blast. However, in demolition of buildings, the disadvantageous noise and shrapnel problems of high strength detonators are even more significant because of the impossibility of burying the units. The reason No. 8 strength caps in detonator units are still used despite these drawbacks is because lower strength detonators of the same design decrease the reliability of initiating signal transmission lines.
If low strength detonators are to be used, the degree of confinement between the signal transmission lines and the detonator must be increased along with the amount of energy transferred to insure reliable initiation. Also, most low strength connection blocks can only accommodate one or two transmission lines. For T and D units to be commercially useful, they must be able to accommodate various numbers of transmission lines connected to the T and D unit. For those connecting blocks that can accommodate several transmission lines, the confinement increases as more transmission lines are connected in the connecting block. However, when a single line is connected to these blocks, many times the confinement is sloppy, and thus another reason for employing a high strength No. 8 cap is to compensate for this lack of confinement. In addition, the connection blocks are designed to operate with higher strength caps. So the confinement block must not only provide enough confinement to insure initiation with a low strength detonator, but it must be versatile enough to accommodate either one or a plurality of signal transmission lines.
The present invention provides a detonator assembly comprising a low strength detonator and a high confinement connection block that effectively initiates a varying number of signal transmission lines, while at the same time eliminating excessive noise and shrapnel. With the elimination of excessive noise and shrapnel there is less likelihood of shrapnel cut off and no need to bury the detonator assembly, thus saving valuable field assembly time. With the ability to initiate a varying number of signal transmission lines, installation is easier, faster and safer with less chance of incorrectly arranging the blasting operation.